The present invention is related to a process for producing lithium carbonate from brine or seawater, and in particular to a process for producing lithium concentrate and lithium carbonate from brine or seawater by combining two concentration techniques of adsorption and electrodialysis.
Due to its merits, such as a high electrical energy density, a high working voltage, a long cyclic life and no memory effect, etc., the lithium ion battery has been widely used in notebook computer, mobile phone, and electric cars, and the use amount thereof will grow in multiples. In addition, many different lithium compounds have been utilized in various industries, such as ceramic industry, glass industry, aluminum electrolysis melting industry and synthetic rubber industry. Among them the most popular lithium compound is lithium carbonate, and many lithium compounds are derived from lithium carbonate. Therefore, lithium carbonate is one of the most important lithium compounds.
Currently, two different sources of raw materials are used to produce lithium carbonate, which are lithium ores and brine. In a country like Taiwan where no nature resource of lithium ores can be found, seawater is the only alternative resource for producing lithium carbonate. Seawater can be used directly or after concentrated, i.e. brine, wherein seawater has a lithium concentration of about 0.1-0.5 ppm, and brine has a lithium concentration of about 10 ppm. In order to form lithium carbonate precipitate from an aqueous solution containing lithium ions, the concentration thereof must be 15000 ppm or higher, which is much higher than those of seawater and brine. Consequently, there is a great interest in developing a technique for raising the lithium ion concentration of the seawater or brine to a level of 15000 ppm required for producing lithium carbonate.
U.S. patent application Ser. No. 10/173,177, which is filed 18 Jun. 2002 and commonly assigned with the present invention, discloses a method for adsorbing lithium ions from a lithium-containing aqueous solution by an adsorbent. The method comprises contacting a lithium-containing aqueous solution to an adsorbent of lithium-containing manganese oxide, so that lithium ions in the aqueous solution are adsorbed on the adsorbent, characterized in that said lithium-containing aqueous solution has a pH value not less than 10. Preferably, said adsorbent of lithium-containing manganese oxide comprises LiMn2O4 or Li2MnO3. The entire content of this application is incorporated herein by reference. The lithium concentrate produced according to the method of this application is restricted to a level of about 1500 ppm. Therefore, there is a need to develop a process for further enriching the lithium concentration from 1500 ppm to about 15000 ppm, a minimum concentration suitable for producing lithium carbonate.
U.S. Pat. No. 4,636,295 discloses a process for producing a lithium enriched solution having a Mg:Li ratio of 5:1 or lower by using a number of electrodialysis steps from brine. In addition, compounds are added to the enriched solution, a filtration operation is required to reduce the amount of Mg ions contained therein, and a final electrodialysis step is carried out to enrich the lithium content to a level of 15.5 g/L, about 1.5%.
A primary object of the present invention is to provide a process for producing lithium concentrate from brine or seawater with a high recovery of lithium ions and free of a solid waste.
In order to accomplish the aforesaid object a process for producing a lithium concentrate from brine according to the present invention comprises the following steps:
a) introducing brine into an adsorbent bed so that lithium ions are more strongly adsorbed on an adsorbent of said adsorbent bed in comparison with ions of Na, K, Ca and Mg;
b)introducing water into said adsorbent bed so that ions of Na, K, Ca and Mg adsorbed on said adsorbent are desorbed therefrom;
c) introducing an acidic solution into said adsorbent bed so that the lithium ions more strongly adsorbed on the adsorbent are desorbed therefrom, and thus a solution having a lithium concentration of 1200-1500 ppm is obtained;
d) enriching lithium ions in the solution having a lithium concentration of 1200-1500 ppm from step c) to a lithium concentration of 6000-10000 ppm by electrodialysis; and
e) enriching lithium ions in the enriched solution having a lithium concentration of 6000-10000 ppm resulting from step d) to a lithium concentration of 14000-18000 ppm by electrodialysis.
In the process of the present invention, a lithium depleted solution having a lithium concentration of 1200-1500 ppm is formed after the electrodialysis in step e), and said process further comprises recycling said lithium depleted solution having a lithium concentration of 1200-1500 ppm to the electrodialysis in step d) as a feed.
In the process of the present invention, a lithium depleted solution having a lithium concentration of 250-400 ppm is formed after the electrodialysis in step d), and said process further comprises enriching lithium ions in said lithium depleted solution having a lithium concentration of 250-400 ppm to a lithium concentration of 1200-1500 ppm by a residue recovery electrodialysis; and recycling the enriched solution having a lithium concentration of 1200-1500 ppm resulting from said residue recovery electrodialysis to the electrodialysis in step d) as a feed. Preferably, a lithium depleted solution having a lithium concentration of 10-100 ppm formed after the residue recovery electrodialysis is recycled to said adsorption bed in step a) as a feed.